1. Field of the Invention
This invention relates generally to amplifier circuits and, more particularly, to an amplifier circuit operating based on a reference voltage, the voltage level of which tends to fluctuate. This invention has a particular applicability to an amplifier circuit provided in a semiconductor integrated circuit.
2. Description of the Background Art
For example, a differential amplifier for amplifying an input signal is often provided in a conventional semiconductor integrated circuit. The differential amplifier has one input connected to receive the input signal and another input connected to receive a prescribed reference voltage.
FIG. 1 is a circuit diagram illustrating an input portion of a conventional semiconductor integrated circuit 2. This integrated circuit 2 comprises a differential amplifier 1 for amplifying an input signal, and a reference voltage generating circuit 3 for applying a prescribed reference direct current voltage (hereinafter referred to as a reference DC voltage) to the differential amplifier 1. The differential amplifier 1 comprises a constant current source 11, PMOS transistors 12 and 14, and NMOS transistors 13 and 15. The transistor 14 has its gate connected to receive the input signal via a pad 82. The transistor 12 has its gate connected to the reference voltage generating circuit 3. An output signal of the differential amplifier 1 is applied to a circuit connected to the succeeding stage, e.g. a comparator. The reference voltage generating circuit 3 comprises resistors 31 and 32 connected in series between a power supply Vcc (=5 V) and the ground (=0 V). This circuit 3 applies an output voltage of 2.5 V as a reference voltage to the amplifier 1. Further, the transistor 12 in the amplifier 1 is connected to receive a signal externally applied via the pad 81. The transistor 12 has its gate connected to a protection circuit formed of diodes 41 and 42 and a resistor 43. The protection circuit protects the transistor 12 from being destroyed by an input signal having its voltage level exceeding a prescribed value, or a surge voltage.
In operation, the reference voltage generating circuit 3 applies the reference voltage of 2.5 V to the differential amplifier 1. The differential amplifier 1 outputs a signal Va amplified responsive to the reference voltage and an input signal Vin externally applied. The output signal Va includes a voltage signal having a reference level of 2.5 V.
However, it could occur that an output voltage from the reference voltage generating circuit 3 frequently fluctuates depending upon the fluctuation of the power supply Vcc. In such a case, the output signal Va of the differential amplifier 1 is affected by the fluctuation of the output voltage of the circuit 3.
A power-supply rejection ratio (hereinafter referred to as PSRR) is, in general, defined as a ratio of a change in an input offset voltage to a corresponding change in one power-supply, with all remaining power voltages held constant. That is, the PSRR shows a change in an offset voltage of the amplifier caused by the fluctuation in the power-supply voltage. As described above, since the reference voltage generating circuit 3 shown in FIG. 1 applies the voltage affected by the fluctuation in the power supply Vcc to the differential amplifier 1, the differential amplifier 1 attains a higher PSRR. That is, it is not preferable because the offset voltage of the differential amplifier 1 fluctuates as per the fluctuation in the power supply Vcc.